In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after Release 12 (see non patent document).
In the D2D communication, a plurality of adjacent user terminals perform direct radio communication in a frequency band assigned to a mobile communication system. It is noted that the D2D communication is also called proximity service (Proximity Service) communication.
A user terminal transmits a discovery-use signal in order to discover a communication partner terminal in the D2D communication. When another user terminal that receives the discovery-use signal transmits a response signal to the user terminal, the user terminal can discover the communication partner terminal in the D2D communication.